Metal treatment



U. c. TAlNToN METAL TREATMENT Feb. 16, 1943.

Original Filed Oct. 2l, 1938 :nausea l lNVENToR Q /6'.' 7v//v7-0/V u l A@wl/M: ATTORNEYS Patented Feb. 16, 1943 l METAL TREATMENT Urlyn CliftonTainton, Baltimore, Md. original application october 21', 193s, serialNo.

Divided and this application March 13, 1942, Serial No. 434,457l

Claims. (Cl. 204-145) My invention relates to the treatment of metal,particularly for the cleaning of the surface thereof. The invention isespecially effective for removing rust, scale, carbon and slag from thesurface 'of iron and steel, and is particularly applicable as apreliminary treatment to various coating operations, such asgalvanizing, enameling, painting, electroplating or the like. Theinvention, however, as will later more fully appear, is not limited tothe treatment of ferrous metal.

My invention comprises the step of subjecting the surfaces of metals tothe reducing action of a metal more electropositive than the metal be'-ingh treated, that is having a position in the electrochemical scaleabove that of the metal being treated. For example, when iron or steelis being treated it may be subjected to the action of sodium or calcium,which metals are considerably more electropositive than iron.

More speciiically, my invention comprises the subjection of the metalarticle to be treated to the action of a more electropositive metal by amethod in which the article is immersed in a bath comprising compoundsof the more electropositive metals under such conditions that a moreelectropositive metal maintained in solution in said bath is active atthe surface of the article.

In presenting this invention, a specific embodiment thereof will firstbe described, followed by illustrative modiications and an outline ofthe general principles upon which it rests.

Accordingly I shall first describe the cleaning of steel or iron wire asa step in the manufacture of electroplated zinc coated wire.

Referring to the drawing:

Fig. 1 is a. diagrammatic elevation, more or less in section, of asystem for cleaning and electroplating the wire; and

, Fig. 2 is a plan view of the system shown in Fig. 1. 1

Referring to Figs. 1 and 2, the wire II is fed continuously from reel I2successively through cleaning tank I3, wash tank I4, anodic cleaningrespectively positive, negative, and positive.

' Guide rolls 25 of electrodes I9 and 20 serve to conduct current towire II, the wire thus beingv rendered anodic in tank I5 and againcathodic in tank I6. Gas burners 29 are provided for maintaining tank I3in a heated condition.

Tank I3 contains a fused bath of sodium hydroxide through which the wirepasses. Current is passed through the fused bath to electrolyze it,electrode 8 serving as the cathode.- In this specific example of myinvention, the wire passes through the bath at such a rate that eachportion thereof remains immersed in the fused bath for a'period of aboutthirty seconds.

From tank I3 the wire passes through wash water in tank I4 to remove theadherent sodium hydroxide, then passing through an aqueous solution ofsulphuric acid in tank I5, the concentration of this being 20% HaSO4.Here the wire acts as the anode, the current density employed being ofthe order of amperes per square foot or greater.

The wire next passes through an aqueous solution of zinc sulphate andsulphuric acid in tank I6, the zinc sulphate being in such an amount asto give a zinc content of 7 grams per 100 cubic centimeters of solutionand the sulphuric acid about 20%. Here the wire acts as the cathode,

the current density being of the order of 200 amperes or greater persquare foot of cathodic surface. Here the wire is electroplated withzinc. For the anodic treatment in tank I5, the *Y same composition maybe employed as that in the coating of plating tank I6.

The anodic treatment of the wire in tank I5 may be effected with anelectrolyte of substantially the same composition as the platingelectrolyte in tank I6. It is to be understood, of course, that the mostsignicant constituent of this electrolyte used in tank I5 is thesulphuric acid. Y

. The treatment in tank I3 is the most significant stage ofthe processso far as this particular patent application is concerned. Thistreatment effectively cleans the surface of the wire, fully removingthose obstacles to effective coating of ferrous metal. Moreover, thistreatment produces a passive condition of the surface of the metal. Thispassivity of the metal surface gives it a considerable degree ofresistance to corrosive influences. y

For instance as shown in the drawing the tank I3 containing fused NaOHmaintained heated by the jets 29 may have its two anodes 2l on the sidesand its cathode 8 on the tank bottom, these electrodes receiving currentfrom the generator 2l and electrolytically supplying elemental sodium tothe caustic bath which is maintained at a temperature sufficiently abovethe fusing point ofthe sodium hydroxide so that the sodium liberated atthe cathode i is dissolved or diffused in the molten caustic at a ratesufficient to supply sodium for the cleaning of the wire. The wire II isrun through the bath under guide rollers 25 unconnected in theelectrical circuit, the holders or brackets I8 being a mere mechanicalsupport for the roller. In this way the wire is submerged in the causticsolution of sodium at a temperature above the fusion point of thecaustic for sufficient time to remove the oxides, scale and the likefrom the wire while in the bath. Upon emerging from the bath the wire isfree of oxides and the like and upon removal of the adhering film ofcaustic the wire surface is conditioned for further treatments referredto in connection with the electrolytically treated metal. The causticmay be replaced by compounds of other alkali or alkaline earth metalsand the various temperature controls may be applied as in theelectrolytic treatment of the article together with the avoidance of theformation of the oxide coating as previously set forth.

While wire has just been given as an example of an article which may beadvantageously treated in sensible amounts upon the article, it ispractically impossible to get uniform reduction of the oxides and othercompounds upon its surface. The sodium collects irregularly upon thesurface,

appearing as globules here and there, and as soon as a portion of thecathode surface receives a film of sodium, the rate and depth ofreduction in that portion is immediately diminished because thecollected sodium is not so effective a reducing agent as nascent sodiumor sodium in solution, and since the i'llm protects the surface from thedirect action of nascent and dissolved sodium. Moreover, the conditionsnecessary for depositing sodium in sensible amounts upon the cathode aresuch that the sodium hydroxide surrounding the article being treatedcontains little, if any, sodium in solution. Consequently, even thoseportions of the article's surface which are not covered by a illm ofsodium are not so quickly and effectively subjected to the action of thesodium.

Another factor which would interefere with getting the best results,when sodium is deposited in sensible amounts upon the article, is thatof reoxidation. Elemental sodium rapidly forms oxides when brought intothe air, the oxides of sodium being very powerful oxidizing agents. Whena ferrous or other metal article, having globules of sodium upon itssurface emerges from the fused bath. oxides of sodium are quickly by theprocess just described, it is evident that formed .which immediatelyreoxidize portions of metal articles generally may be treated by thecleaning operation in fused sodium hydroxide whether the articles are tobe fed continuously through the fused bath, as in the case of wire, or'

the articles surface.

The most easily imposed conditions for effecting the treatment withsodium without collecting sensible quantities of sodium on the surfaceof are to be treated intermittent1y Obviously, the 35 the article beingtreated is by control of the temapplication of the fused bath treatmentis not limited to articles which are to be zinc coated as in the specicillustration just given. The method is applicable wherever it is desiredto obtain an effectively cleaned metal surface. This type of cleaning isespecially effective as a preparation of metal surfaces for subsequentcoating operation, whether electrolytic or otherwise, includingenameling and the like, but its application is as wide as the need foreffectively cleaned metal surfaces.

The cleaning effect produced by the process set forth above is due tothe reducing action of the elemental sodium on the compounds, such asogrides, on the surface of the iron or steel being treated.

For the, most effective results, the treatment with the fused sodiumhydroxide should be so carried out that the elemental sodium liberateddoes not collect in sensible'amounts, preferably not in visible amounts,upon the article being treated. To effect this result, I employconditions during electrolysis such that the sodium as fast as it isseparated from the sodium hydroxide, in the elemental state. isdissolved by or diffused into the bath of sodium hydroxide, andconsequently does not gather or collect upon the surface of the cathode.

Under such conditions the article is bathed by a solution of sodium insodium hydroxide which acts reducingly upon any compounds on thearticles surface.

If conditions were to be used which would cause the collection ofsensible amounts of sodium on the cathode, as is done in the knownprocesses for electrolytic recovery of metallic sodium, the action ofthe sodium upon the article being treated would not be so effective asvwhen the procedure just outlined is followed. If sodium is depositedperature of the fused bath of sodium hydroxide. I maintain this fusedbath at a temperature above that at which sodium collects upon thecathode. By keeping the bath at temperatures more than 20 C. in excessof the melting point of sodium hydroxide, the sodium is prevented fromcollecting on the cathode.

Preferably I employ the sodium hydroxide bath at a minimum temperatureof 350 C., but, as implied above, I may go as low as a temperature justin excess of 20 above the meltipg point of the sodium hydroxide.Consequently'I may use a temperature as low as just in excess of 338 C.in the case of pure sodium hydroxide. Since most commercial sodiumhydroxides are more or less impure, their melting points are lower thanthat of the pure hydroxide, ordinarily ranging from about 295 C. to 300C.; consequently with these impure sodium hydroxides I may operate atsomewhat lower temperatures, if desirable, namely at temperatures whichare just in excess of 315 C. to 320 C.

Thus far we have considered the minimum temperature at which the fusedbath of sodium hydroxide is used. It is frequently desirable to operateat fairly high temperatures and this may be successfully done. The fusedbath may combine the function of annealing the metal articles with thatof its cleaning action thereon. Steel wire, for example, is commonlyproduced by being "cold drawn" which operation introduces more or lessstresses or strains. By operating the fused bath at a sufficientlyelevated temperature these conditions may be entirely relieved. Withthis additional function of annealing, or for that matter any desiredform of heat treatment, the fused bath will be used at temperatures bestadapted to the particular material and purpose involved.

High chromiumsteels, including the so-called "stainless steels? such asthe weli known 1B-8 (18% chromium and 8% nickel), are advantageouslytreated by my cleaning process. In treating these high chromium steelsit is' usually well to employ temperatures of the order of 538 C. A veryimportant aspect of my invention relatesto the control of conditions toavoid or nullify the disadvantage of reoxidation of the surface ofthemetal being treated. I have discovered that unless certain precautionsare taken the metal quickly oxidizes upon removal from the cleaning anddeoxldizing bath. If, in the oper-t A ation of the process given aboveas an example of my invention, the bath of sodium hydroxide ismaintained at a temperature substantially above 550 C. the surface ofthe iron or steel becomes oxidized when it passes from the fused sodiumAhydroxide in tank Il into the air. I'his condition can frequentlybe`obviated by maintaining the fused'bath of sodium hydroxide at atemperature below 550 C., at 500 C., for example. By using the cleaningbath at this temperature the oxidation of the treated metal is usuallyavoided by preventing at the outset the condition of the steel surfacewhich leads to oxidation. Asv wil be apparent later in thisspecification, it is feasible under certain circumstances to operate thefused cleaning bath at temperatures which normally render the metaleasily oxidizable and then to subject the metal being treatedimmediately to conditions which will overcome the tendency to oxidize.

In carrying out the process above described, it may be useful in somecases to employ temperatures of the fused bath in excess of 550 C. Itmay be desired, for example, to effect an annealing of wire in the fusedbath at a temperature say of 600 C. Ifl such a temperature of the fusedbath is employed in tank I3 the surface of the wire will oxidize when itenters the air. The oxides on the surface, thus produced, however, arenot so closely adherent as those which normallyl occur on ferroussurfaces and they may easily be removed by a relatively mild picklingoperation in an aqueous acid bath, such a pickling operation being muchmore easily and inexpensively carried out than if the metal had notpreviously been subjected to the fused bath treatment. Of course, whenthe article, after its treatment in the sodium hydroxide bath, is giventreatment such as the anodic treatment illustrated in Figs. l and 2, theoxides are effectively removed. I

But while it is practicable to thus remove the oxides formed on the wireor other metal I prefer in most cases to proceed in such manner as toprevent their occurrence. In the process involving the anodic treatmentthe removal of the oxides by the anodic operation has a disadvantageinthat to effect their complete removal it is usually necessary toprolongthe anodic treatment thus slowing up the entire'process.Accordingly when I employ such temperatures of the fused bath as producethe tendency to ready oxidability I find it advantageous to use someform of procedure which will nullify this tendency. For instance I mayemploy a type of furnace for heating the pot which permits concentrationof heat at the input end and allows the temperature at the exit end toremain considerably lower.

It is possible to secure the combined annealing and cleaning in thehigher temperature portion of the bath, and by passing the wire or othermetal article through the lower temperature porfused bath serves toovercome the tendency of steam, the ltendency to oxidation may be subthemetal to oxidize. Even in the absence of means for cooling the exitportion of the bath and of the treatment of the metal and bath withstantially or even completely overcome if the fused bath is sufficientlyagitated.

Other means for preventing the rapid oxidation of the metal as it leavesthe fused cleaning bath may be employed. 'I'he oxygen in the air appearsto bea factor in the reoxidation of the metal as it leaves the fusedbath. By providing an atmosphere of a non-oxidizing gas, particularlyone having a reducing action, reoxidation is substantially or entirelyprevented.

Instead of working with the higher temperatures of the bath which givethe tendency to active oxidation, it is of course practicable to employthe lower temperatures and effect an annealing by previously passing thewire or other article through a lead bath or other suitable medium atthe appropriate annealing temperatures. 1

My process is not-limited in its application to sodium hydroxide. Othercompounds of sodium may be used, or mixtures of different compounds.

y Nor is my process limited to the use of compounds of sodium. Compoundsof the other alkali metals may be used, those of potassium beingparticularly suitable. I may. also use compoundsv of the alkaline earthmetal group including magnesium, those of calcium being economicallyadvantageous. Hereafter, when referring generically tok thesevarious.metals and their com'- pounds, I shall usually designate them ashighly electropositive metals or compounds thereof.

Not only with sodium hydroxide but with other compounds of sodium aswell as compounds of vthe other metals of the alkali metal group themetals of the alkaline earth group, I prefer to operate at temperaturessubstantially above their melting points. I find that the reducingaction of these highly electropositive metals is much more eiectiveifthe operating temperature of the bath i's substantially in excess of themelting point of the compound or mixture of compounds which is used.Ordinarily the operating temperature of the bath should be in excess oftwenty degrees above the melting point of the bath. By so proceeding, Ifind that I avoid the superficial reduction of compounds which may occurif lower temperatures are used, moreover the resulting.

reduced metal is much more easily detached than if the lowertemperatures were to be used.

By selection of the proper compound for the fused bath or by a mixtureof compounds-a wide varietyof effects may be produced, thus enabling oneto select the conditions most suitable for his particular material andproblem. For example, if lower temperatures of operation, than thosesecurable by the use of sodium hydroxide, are desired, such temperaturescan be obtained by the use of compounds of lower melting points, such,for example, as sodium nitrite (fusing at 213 C.) either alone or mixedwith sodium hydroxide. By the judicious mixing of various sodium andpotassium salts, as is well known, a rather wide range of fusion pointsmay be'secured;

It will be obvious to the electro-chemist that the working conditionsmust frequently be altered with a change in the character of the fusedbath employed. If a chloride of a highly electropositive metal were tobe used, for example, it would be necessary that the -anodes used shouldb'e of some material resistant to the action of chlorine.

Usually in carrying out my cleaning operation it is important tomaintain the fused bath relatively constant in composition or at leastto allow variations only within definite limits. If there is unduevariation in the composition of the fused bath the temperature will varyunduly. If, for example, sodium hydroxide is used for the bath undueformation of sodium carbonate through absorption from the air of carbondioxide will lead to such an elevation of the melting point of the bathas to give bath temperatures too high for the most satisfactoryoperation of my process. Where, for instance, it is desired to operatethe bath of sodium hydroxide at a temperature below 550 C. to preventthe ready oxidability of the metal, it is necessary that means beprovided to prevent the absorption of such an Iamount of carbon dioxideas will raise the melting point above the desired operating temperature.The carbonate content of the fused bath will of course depend upon therate of removal of the mixture of sodium hydroxide and sodium carbonateupon the surface of the metal being treated, the rate of replenishmentof the fused bath by the addition of pure sodium hydroxide, and the rateof absorption of carbon dioxide. The most effective way of keeping thecarbon dioxide absorption at the necessary minimum is Iby providing acover for the fused bath cleaning tank.

The loss of sodium hydroxide, or other bath constituent, from the fusedbath, due to its being carried away on the surface of the .articletreated, may be minimized in various ways as by wiping the wire or otherarticle as it issued from the bath.

As la result of the article being surrounded by a solution of thereducing metal, all portions of the articles surface can be effectivelytreated.

I am thus able to reduce oxides, and other compounds, effectively whenthe article being treated does not act as cathode. By producing asolution of the highly electro-positive metal an article in the fusedbath may be subjected to the reducing action'of the solution even thoughsuch article is in no way electrically connected with the cathode.

Accordingly, my invention, in its broader aspect, includes the idea ofsubjecting metal articles to a solution of highly electro-positive metalin a fused bath. For example, articles having highly oxidized surfacesmay be effectively cleaned by immersing them for a substantial time in asolution of sodium in fused sodium hydroxide, regardless of how thesolution is pren pared, whether electrolytically or otherwise. The

solution obviously may be prepared in various ways. Elemental sodium maybe gradually added to a fused bath of sodium hydroxide in any lmannerdesired. Probably the most effective way tomaintain a solution of sodiumin the hydroxide My process may be effectively utilized to producesponge metal. For example, my process is very effective in treating allferrous material which has exceedingly heavy amounts of scale. When suchmaterial is subjected to my operation, as for instance, when treated ina caustic soda bath, as above outlined, the sodium, in solution in thecaustic soda. quickly and thoroughly reduces the heavy scale to spongeiron. This sponge iron may be easily removed by Well known operations.such as scraping. and by the use of water sprays. The sponge iron thusremoved is an important industrial product and it is frequentlyeconomically advantageous to utilize such material where heavilyoxidized material is treated on a large scale.

I have already indicated that the fused bath may be so used as tocombine with its function of cleaning that of effecting the heatingstage of heat treatments, such as annealing. These heat treatments arevarious. Many of them, such as most forms of normalizing include thestep of heating the metal above the critical temperature. The fusedbath, which is used for cleaning the metal, may well be employed is byintroducing it in the manner already pointedf out, namely byelectrolyzing a solution of hydroxide at a-sumciently elevated tempe tocause the liberated sodium to fused bath.

My process is not limited to the treatmentof ferrous metals. Articles of-various metals may be efficiently cleaned by subjecting them to the i7o diffuse the for the heating step of such heat treating operations. Ifthe-desired temperature is a very high one care should be taken toemploy such a compound as will not react with the metal being treated atthe elevated temperature. For example, I have foundthat in the combinedheat treatment and cleaning of ferrous articles a mixture of 50%potassium chloride and 50% sodium carbonate may be used at considerablyhigher temperaturesthan sodium hydroxide.

Frequently I find it advantageous to subiect the article to be cleanedto an oxidizing treatlment prior to the reducing action of the highlyelectropositive metal. l

'I'he treatment of the article without current accomplishes thedeoxidation of the oxides, scale and the like within the bath, and it isunderstood that the treatment without current may be preceded by andfollowed by the electrolytic and oxide-preventing treatments.

In the specification and claims, electrode means either terminal of anelectric source by which the current enters or leaves the electrolyte.

'I'his application is a division of my co-pending application Serial No.236,340, filed October 21, 1938, which application is in part acontinuation of -application Serial No.l 630,233, nled August 24,1932,'application Serial No. 692,378, filed October 6, 1933, andapplication Serial No. 128,682,

Ytion a metal also from said group and electropositive relative to thetreated metal of the ar` ticle, introducing said article in solid forminto said bath without connection of said article as an electrode of anelectric source supplying current to the bath, said article being solidat the temperature of the bath,v maintaining said bath at a temperaturebelow the boiling point of said treating metal and suilciently highabove the fusion point of the molten compound to maintain said suppliedmetal in solution in said bath at the area of contact with said articlewhereby to avoid a deposit of said treating metal on said article,continuing said treatment for a suicient length of time with suicienttreating metal in solution and without connection of the article as anelectrode of an electric source to complete the desired reduction of theoxides, scale and the like in the bath without dependence on currentthrough the article as an electrode of an electric source and withoutaccumulation of deposit of said treating metal on the article, andremoving the article from the molten bath.

2. The process of claim 1 in which oxides are incompletely reduced inthe bath and the article after delivery is subjected to treatment in anaqueous acid bath to remove oxides adhering to the surface of thedelivered article.

3. 'Ihe process of claim 1 in which the article at delivery has beensubjected to surface oxidation and is subsequently treated with anaqueous acid bath to remove the adhering oxides.

4. The process of claim 1 in which the treated metal is freed fromactively oxidizing material by a fluid bath on the treated metal beforesaid metal is brought into the free atmosphere.

5. The process of claim 1 in which the .treated metal at the deliverysurface of the molten bath is bathed in an atmosphere of steam.

6. The process of claim 1 in which the bath is stirred to agitate it atthe point of delivery of the article therefrom.

7. The process of claim 1 in which the fused bath is of a solution ofsodium in sodium hydroxide maintained at a temperature between 350 C.and '750 C.

8. The process of claim 1 in which the bath contains a treating metal ofthe group sodium, calcium, potassium and magnesium.

9. The process of claim 1 in which the desired amount of treating metalin solution is maintained by the addition of said treating metal inelemental form from time to time.

10. The process of claim 1 in which the supply of metal for the moltenbath is from a cathode suspended in the bath and in electrical circuitwith an anode in the bath.

URLYN CLIFTON TAINTON.

